Wireless Sensor Having a Variable Transmission Rate

ABSTRACT

A wireless battery-powered daylight sensor for measuring a total light intensity in a space is operable to transmit wireless signals using a variable transmission rate that is dependent upon the total light intensity in the space. The sensor comprises a photosensitive circuit, a wireless transmitter for transmitting the wireless signals, a controller coupled to the photosensitive circuit and the wireless transmitter, and a battery for powering the photosensitive circuit, the wireless transmitter, and the controller. The photosensitive circuit is operable to generate a light intensity control signal in response to the total light intensity in the space. The controller transmits the wireless signals in response to the light intensity control signal using the variable transmission rate that is dependent upon the total light intensity in the space. The variable transmission rate may be dependent upon an amount of change of the total light intensity in the space. In addition, the variable transmission rate may be further dependent upon a rate of change of the total light intensity in the space.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of commonly-assigned U.S.patent application Ser. No. 13/875,434, filed May 2, 2013, entitledWIRELESS BATTERY-POWERED DAYLIGHT SENSOR, which is a continuationapplication of commonly-assigned U.S. patent application Ser. No.12/727,956, filed Mar. 19, 2010, now U.S. Pat. No. 8,451,116, issued May28, 2013, entitled WIRELESS BATTERY-POWERED DAYLIGHT SENSOR, which is anon-provisional application of U.S. Provisional Patent Application Ser.No. 61/164,098, filed Mar. 27, 2009, entitled METHOD OF CALIBRATING ADAYLIGHT SENSOR; U.S. Provisional Patent Application Ser. No.61/174,322, filed Apr. 30, 2009, entitled WIRELESS BATTERY-POWEREDDAYLIGHT SENSOR; and U.S. Provisional Patent Application Ser. No.61/285,628, filed Dec. 11, 2009, entitled WIRELESS BATTERY-POWEREDDAYLIGHT SENSOR. The entire disclosures of each of these non-provisionaland provisional patent applications are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to daylight sensors for measuring theambient light level (i.e., the total light intensity) in a space, andmore particularly, to a lighting control system having a lightingcontrol device (such as a dimmer switch) and a wireless, battery-powereddaylight sensor.

2. Description of the Related Art

Many rooms in both residential and commercial buildings are illuminatedby both artificial light from a lighting load, such as an incandescentlamp or a fluorescent lamp, and daylight (i.e., sunlight) shiningthrough a window. Daylight sensors (i.e., photosensors) are often usedto measure the total light intensity in a space in order to adjust thelight intensity of the lighting load to thus adjust the total lightintensity in the space. For example, the light intensity of the lightingload may be decreased as the total light intensity increases, and viceversa. Daylight sensors are typically mounted to a ceiling in the spaceat a distance from the window. Since electrical wires (for power andcommunication) are typically not located near the position on theceiling to which the daylight sensor must be mounted, it is desirablethat the daylight sensor be “wireless” in order to avoid the need to runelectrical wires to the daylight sensor (for example, in a retro-fitinstallation). Therefore, there is a need for a battery-powered daylightsensor that is able to communicate wirelessly with a load controldevice, such as a dimmer switch.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, a wirelessbattery-powered daylight sensor for measuring a total light intensity ina space is operable to transmit wireless signals using a variabletransmission rate that is dependent upon the total light intensity inthe space. The sensor comprises a photosensitive circuit, a wirelesstransmitter for transmitting the wireless signals, a controller coupledto the photosensitive circuit and the wireless transmitter, and abattery for powering the photosensitive circuit, the wirelesstransmitter, and the controller. The photosensitive circuit is operableto generate a light intensity control signal in response to the totallight intensity in the space. The controller transmits the wirelesssignals in response to the light intensity control signal using thevariable transmission rate that is dependent upon the total lightintensity in the space. The variable transmission rate may be dependentupon an amount of change of the total light intensity in the space. Inaddition, the variable transmission rate may be further dependent upon arate of change of the total light intensity in the space.

According to another embodiment of the present invention, a wirelessbattery-powered daylight sensor for measuring a total light intensity ina space comprises a photosensitive circuit operable to generate a lightintensity control signal in response to the total light intensity in thespace, a wireless transmitter for transmitting wireless signals, acontroller coupled to the photosensitive circuit and the wirelesstransmitter, and a battery for powering the photosensitive circuit, thewireless transmitter, and the controller. The controller is operable totransmit a wireless signal in response to the light intensity controlsignal, and is operable to disable the photosensitive circuit, such thatthe photosensitive circuit does not draw current from the battery. Inaddition, the photosensitive circuit may comprise a photosensitive diodefor conducting a photosensitive diode current having a magnituderesponsive to the light intensity in the space, where the magnitude ofthe light intensity control signal is responsive to the magnitude of thephotosensitive diode current. The photosensitive circuit may furthercomprise a controllable switch coupled in series with the photosensitivediode, such that the photosensitive diode conducts the photosensitivediode current when the switch is closed. The controller may be coupledto the switch for opening the switch, such that the photosensitive diodedoes not conduct the photosensitive diode current and the photosensitivecircuit is disabled.

According to yet another embodiment of the present invention, a wirelessbattery-powered daylight sensor for measuring a total light intensity ina space operates as part of a lighting control system that comprises adimmer switch for controlling the amount of power delivered to alighting load. The sensor comprises a photosensitive circuit operable togenerate a light intensity control signal in response to the total lightintensity in the space, a wireless transmitter for transmitting wirelesssignals, a controller coupled to the photosensitive circuit and thewireless transmitter, and a battery for powering the photosensitivecircuit, the wireless transmitter, and the controller. The controller isoperable to determine, in response to the light intensity controlsignal, a new light intensity to which the dimmer switch should controlthe intensity of the lighting load. The controller is further operableto enable the wireless transmitter and to transmit to the dimmer switcha wireless signal including a command that includes the new lightintensity for the lighting load if the new light intensity differs froma present light intensity of the lighting load by a predeterminedincrement.

According to another aspect of the present invention, a wirelessbattery-powered daylight sensor for measuring a total light intensity ina space comprises a photosensitive circuit operable to generate a lightintensity control signal in response to the total light intensity in thespace, a wireless transceiver for transmitting and receiving wirelesssignals, a laser pointer circuit adapted to be exposed to light from alaser pointer, a controller coupled to the photosensitive circuit, thewireless transceiver, and the laser pointer circuit, and a battery forpowering the photosensitive circuit, the wireless transceiver, and thecontroller. The controller is operable to transmit a wireless signal inresponse to the light intensity control signal. The controller isfurther operable to enable the wireless transceiver in response to lightfrom a laser pointer shining on the laser pointer circuit, and tosubsequently receive a wireless signal.

Other features and advantages of the present invention will becomeapparent from the following description of the invention that refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail in the followingdetailed description with reference to the drawings in which:

FIG. 1 is a simple diagram of a radio-frequency (RF) lighting controlsystem comprising a dimmer switch and a daylight sensor according to thepresent invention;

FIG. 2 is a simplified diagram of a room in which the daylight sensor ofFIG. 1 may be mounted;

FIG. 3 shows a few example plots of total light intensities at thedaylight sensor mounted in the room of FIG. 2 with respect to timeduring a sunny day, a cloudy day, and an intermittent-cloudy day;

FIG. 4 is an enlarged perspective view of the daylight sensor of FIG. 1;

FIG. 5 is a simplified block diagram of the dimmer switch of FIG. 1;

FIG. 6A is a simplified block diagram of the daylight sensor of FIG. 1;

FIG. 6B is a simplified schematic diagram of the daylight sensor of FIG.6A;

FIG. 7 is a simplified flowchart of a transmission algorithm executed bya controller of the daylight sensor of FIG. 1 according to a firstembodiment of the present invention, such that the daylight sensortransmits digital messages using a variable transmission rate;

FIG. 8 is a simplified flowchart of a variable transmission controlprocedure executed by the controller of the daylight sensor of FIG. 1according to the first embodiment of the present invention;

FIG. 9 is a simplified flowchart of a receive procedure executed by acontroller of the dimmer switch of FIG. 1 according to the firstembodiment of the present invention;

FIG. 10A is a simplified flowchart of a laser pointer interruptprocedure executed by the controller of the daylight sensor of FIG. 1;

FIG. 10B is a diagram of an example test setup for the daylight sensorof FIG. 1 according to the first embodiment of the present invention;

FIG. 10C is a plot of an example test waveform for the daylight sensorof FIG. 1 according to the first embodiment to be used in the test setupshown in FIG. 10B;

FIG. 11 is a simplified flowchart of a variable transmission controlprocedure executed by the controller of the daylight sensor of FIG. 1according to a second embodiment of the present invention;

FIG. 12 is a simplified flowchart of a receive procedure executed by thecontroller of the dimmer switch of FIG. 1 according to the secondembodiment of the present invention;

FIG. 13 is a simplified flowchart of a load control procedure executedperiodically by the controller of the dimmer switch of FIG. 1 accordingto the second embodiment of the present invention;

FIG. 14 is a simplified flowchart of a variable transmission controlprocedure executed by the controller of the daylight sensor of FIG. 1according to a third embodiment of the present invention;

FIG. 15 is a simplified flowchart of a transmission algorithm executedby the controller of the daylight sensor of FIG. 1 according to a fourthembodiment of the present invention;

FIG. 16A is a simplified flowchart of a variable transmission controlprocedure executed by the controller of the daylight sensor of FIG. 1according to the fourth embodiment of the present invention;

FIG. 16B is a plot of an example test waveform for the daylight sensorof FIG. 1 according to the fourth embodiment to be used in the testsetup shown in FIG. 10B; and

FIG. 17 is a simplified flowchart of a control procedure executedperiodically by the controller of the daylight sensor of FIG. 1according to a fifth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description ofthe preferred embodiments, is better understood when read in conjunctionwith the appended drawings. For the purposes of illustrating theinvention, there is shown in the drawings an embodiment that ispresently preferred, in which like numerals represent similar partsthroughout the several views of the drawings, it being understood,however, that the invention is not limited to the specific methods andinstrumentalities disclosed.

FIG. 1 is a simple diagram of a radio-frequency (RF) lighting controlsystem 100 comprising a dimmer switch 110 and a daylight sensor 120according to a first embodiment of the present invention. The dimmerswitch 110 is adapted to be coupled in series electrical connectionbetween an alternating-current (AC) power source 102 and a lighting load104 for controlling the amount of power delivered to the lighting load.The dimmer switch 110 may be wall-mounted in a standard electricalwallbox. Alternatively, the dimmer switch 110 could be implemented as atable-top load control device. The dimmer switch 110 comprises afaceplate 112 and a bezel 113 received in an opening of the faceplate.The dimmer switch 110 further comprises a control actuator 114 (i.e., abutton) and an intensity adjustment actuator 116. Successive actuationsof the toggle actuator 114 toggle, i.e., turn off and on, the lightingload 104. Actuations of an upper portion 116A or a lower portion 116B ofthe intensity adjustment actuator 116 respectively increase or decreasethe amount of power delivered to the lighting load 104 and thus increaseor decrease a present light intensity L_(PRES) of the lighting load 104from a minimum intensity (e.g., 1%) to a maximum intensity (e.g., 100%).A plurality of visual indicators 118, e.g., light-emitting diodes(LEDs), are arranged in a linear array on the left side of the bezel113. The visual indicators 118 are illuminated to provide feedback ofthe intensity of the lighting load 104. An example of a dimmer switchhaving a toggle actuator 114 and an intensity adjustment actuator 116 isdescribed in greater detail in U.S. Pat. No. 5,248,919, issued Sep. 29,1993, entitled LIGHTING CONTROL DEVICE, the entire disclosure of whichis hereby incorporated by reference.

The daylight sensor 120 is mounted so as to measure a total lightintensity L_(T-SNSR) in the space around the daylight sensor (i.e., inthe vicinity of the lighting load 104 controlled by the dimmer switch110). The daylight sensor 120 includes an internal photosensitivecircuit, e.g., a photosensitive diode 232 (FIG. 6A), which is housed inan enclosure 122 having a lens 124 for conducting light from outside thedaylight sensor towards the internal photosensitive diode 232. Thedaylight sensor 120 is responsive to the total light intensityL_(T-SNSR) measured by the internal photosensitive circuit.Specifically, the daylight sensor 120 is operable to wirelessly transmitdigital messages (i.e., wireless signals) to the dimmer switch 110 viaRF signals 106, such that the dimmer switch 110 controls the presentlight intensity L_(PRES) of the lighting load 104 in response to thetotal light intensity L_(T-SNSR) measured by the daylight sensor 120.

During a setup procedure of the RF lighting control system 100, thedaylight sensor 120 may be assigned to (i.e., associated with) thedimmer switch 110. As mentioned above, the daylight sensor 120 transmitsdigital messages wirelessly via the RF signals 106 to the dimmer switch110 in response to the total light intensity L_(T-SNSR) measured by thedaylight sensor. A digital message transmitted by the daylight sensor120 includes, for example, identifying information, such as, a serialnumber (i.e., a unique identifier) associated with the daylight sensor.The dimmer switch 110 is responsive to messages containing the serialnumbers of the daylight sensor 120 to which the dimmer switch isassigned. Each digital message may further comprise a valuerepresentative of the measured total light intensity L_(T-SNSR) measuredby the daylight sensor 120 (e.g., in foot-candles). Accordingly, thedimmer switch 110 controls the present light intensity L_(PRES) of thelighting load 104 in response to receiving a digital message with thetotal light intensity L_(T-SNSR) as measured by the daylight sensor 120.According to the present invention, the daylight sensor 120 is operableto transmit digital messages to the dimmer switch 110 using a variabletransmission rate f_(TX) that is dependent upon the measured total lightintensity L_(T-SNSR), such that the daylight sensor 120 only transmitsdigital messages when needed (as will be described in greater detailbelow).

Examples of RF lighting control systems are described in greater detailin U.S. patent application Ser. No. 12/033,223, filed Feb. 19, 2008,entitled COMMUNICATION PROTOCOL FOR A RADIO-FREQUENCY LOAD CONTROLSYSTEM; U.S. patent application Ser. No. 12/203,518, filed Sep. 3, 2008,entitled RADIO-FREQUENCY LIGHTING CONTROL SYSTEM WITH OCCUPANCY SENSING;U.S. patent application Ser. No. 12/203,500, filed Sep. 3, 2008,entitled BATTERY-POWERED OCCUPANCY SENSOR; and U.S. patent applicationSer. No. 12/371,027, filed Feb. 13, 2009, entitled METHOD AND APPARATUSFOR CONFIGURING A WIRELESS SENSOR, the entire disclosures of which arehereby incorporated by reference.

Alternatively, the dimmer switch 110 could be replaced with anelectronic switch comprising, for example, a relay, for simply togglingthe lighting load 104 on and off. The electronic switch could be adaptedto simply turn the lighting load 104 on when the measured total lightintensity L_(T-SNSR) drops below a predetermined threshold and turn thelighting load off when the measured total light intensity L_(T-SNSR)rises above approximately the predetermined threshold (e.g., using somehysteresis).

The lighting control system 100 could additionally comprise one or moremotorized window treatments, such as roller shades, draperies, Romanshades, or blinds, for controlling the amount of daylight entering thespace around the daylight sensor 120. Examples of load control systemshaving motorized window treatments are described in greater detail inU.S. Pat. No. 7,111,952, issued Sep. 26, 2006, entitled SYSTEM TOCONTROL DAYLIGHT AND ARTIFICIAL ILLUMINATION AND SUN GLARE IN A SPACE,the entire disclosure of which is hereby incorporated by reference.

FIG. 2 is a simplified diagram of a room 130 in which the daylightsensor 120 may be mounted. The daylight sensor 120 is mounted to aceiling 132 of the room 130 at a distance from a window 134 throughwhich natural light (i.e., daylight) shines. The lighting load 104 isalso mounted to the ceiling 132 of the room. The room 130 contains atask surface 136 (e.g., a table) that is illuminated by the naturallight shining through the window 134 and the electric light (i.e.,artificial light) generated by the lighting load 104. Thus, a totallight intensity L_(T-TASK) produced on the task surface 136 is the sumof a light intensity L_(D-TASK) on the task surface from only daylightentering the room 130 through the window 134 and a light intensityL_(E-TASK) on the task surface from only the lighting load 104 (i.e.,L_(T-TASK)=L_(D-TASK)+L_(E-TASK)). The daylight sensor 120 is operableto measure the total light intensity L_(T-SNSR) at the daylight sensor,which is also a combination of the natural light and the electric lightin the room 130. The natural and electric light that shine onto the tasksurface 136 may be reflected to the daylight sensor 120, while thenatural light from the window 134 may shine directly onto the daylightsensor. Thus, the total light intensity L_(T-SNSR) measured by thedaylight sensor 120 is the sum of a light intensity L_(D-SNSR) at thedaylight sensor from only daylight entering the room 130 through thewindow 134 and a light intensity L_(E-SNSR) at the daylight sensor fromonly the lighting load 104 (i.e., L_(T-SNSR)=L_(D-SNSR)+L_(E-SNSR)).

The dimmer switch 110 adjusts the present light intensity L_(PRES) ofthe lighting load 104 so as to control the total light intensityL_(T-TASK) on the task surface 136 towards a target total task surfacelight intensity L_(TRGT-TASK). For example, the target total tasksurface light intensity L_(TRGT-TASK) may be preset to be approximatelyfifty foot-candles. In addition, the target total task surface lightintensity L_(TRGT-TASK) may be decreased by actuating the intensityadjustment actuator 116. Alternatively, the dimmer switch 110 could beoperable to receive one or more digital messages from an advancedprogramming device, such as a personal digital assistant (PDA) or apersonal computer (PC), such that the target total task surface lightintensity L_(TRGT-TASK) may be entered using a graphical user interface(GUI) and transmitted to the dimmer switch 110. Further, the targettotal task surface light intensity L_(TRGT-TASK) could alternatively beadjusted using an advanced programming mode of the dimmer switch 110. Anexample of an advanced programming mode for a dimmer switch is describedin greater detail in U.S. Pat. No. 7,190,125, issued Mar. 13, 2007,entitled PROGRAMMABLE WALLBOX DIMMER, the entire disclosure of which ishereby incorporated by reference.

Since the total light intensity L_(T-SNSR) measured by the daylightsensor 120 (e.g., as reflected on the daylight sensor) is less than thetotal light intensity L_(T-TASK) shining directly on the task surface136, the lighting control system 100 is characterized by one or moregains. Specifically, the dimmer switch 110 uses a daylight gain G_(D)and an electrical light gain G_(E) to control the present intensityL_(PRES) of the lighting load 104. The daylight gain G_(D) isrepresentative of the ratio between the light intensity L_(D-TASK) onthe task surface 136 from only daylight and the light intensityL_(D-SNSR) measured by the daylight sensor 120 from only daylight (i.e.,G_(D)=L_(D-TASK)/L_(D-SNSR)). The electric light gain G_(E) isrepresentative of the ratio between the light intensity L_(E-TASK) onthe task surface 136 from only the lighting load 104 and the lightintensity L_(E-SNSR) measured by the daylight sensor 120 from only thelighting load (i.e., G_(E)=L_(E-TASK)/L_(E-SNSR)). The daylight gainG_(D) and the electrical light gain G_(E) of the lighting control system100 are set during a gain calibration procedure. An example of a gaincalibration procedures are described in greater detail incommonly-assigned, co-pending U.S. patent application Ser. No.12/727,923, filed Mar. 19, 2010, entitled METHOD OF CALIBRATING ADAYLIGHT SENSOR, the entire disclosure of which is hereby incorporatedby reference.

During days when there are intermittent clouds passing the building inwhich the room 130 is located, the total light intensity L_(T-SNSR) atthe daylight sensor 120 may fluctuate between high values when theclouds are not blocking the sunlight and low values when the clouds areblocking the sunlight. FIG. 3 shows a few example plots of the totallight intensity L_(T-SNSR) as measured by the daylight sensor 120 withrespect to time during a sunny day, a cloudy day, and anintermittent-cloudy day. The total light intensity L_(T-SNSR) during aday typically takes the shape of a parabola. On a sunny day, a totalsunny-day light intensity L_(T-SUNNY) may increase from sunrise (at timet_(SUNRISE)) to a maximum sunny-day light intensity L_(MAX-SUNNY) aroundmidday (at time t_(MIDDAY)), and then decrease until sunset (at timet_(SUNSET)). On a cloudy day, a total cloudy-day light intensityL_(T-CLOUDY) may increase from sunrise to a maximum cloudy-day lightintensity L_(MAX-CLOUDY) around midday, and then decreases until sunset.The maximum sunny-day light intensity L_(MAX-SUNNY) is typically greaterthan the maximum cloudy-day light intensity L_(MAX-CLOUDY). On a dayhaving intermittent clouds, a total light intensity L_(T-IC) mayfluctuate between the total cloudy-day light intensity L_(T-CLOUDY) andthe total sunny-day light intensity L_(T-SUNNY) as shown in FIG. 3.

FIG. 4 is an enlarged perspective view of the daylight sensor 120. Thelens 124 is transparent such that the light from the room 130 is able toshine onto the internal photosensitive diode 232 of the daylight sensor120. The daylight sensor 120 is positioned on the ceiling 132 such thatan arrow 140 points towards the window 134, such that lens 124 isdirected towards the window 134. As a result, more natural light thanartificial light will shine through the lens 124 and onto the internalphotosensitive diode 232. A plurality of actuators (e.g., a calibrationbutton 150, a test button 152, and a link button 154) are used duringthe setup and calibration procedures of the daylight sensor 120. Thedaylight sensor 120 further comprises a laser-pointer receiving opening156, which is adapted to receive energy from a laser pointer (notshown). The daylight sensor 120 is responsive to the energy of the laserpointer shining through the laser-pointer receiving opening 156. Whenthe daylight sensor 120 is mounted to the ceiling 132, a user may shinethe laser pointer through the opening 156 rather than actuating thecalibration button 150 during the gain calibration procedure.

FIG. 5 is a simplified block diagram of the dimmer switch 110. Thedimmer switch 110 comprises a controllably conductive device 210 coupledin series electrical connection between the AC power source 102 and thelighting load 104 for control of the power delivered to the lightingload. The controllably conductive device 210 may comprise any suitabletype of bidirectional semiconductor switch, such as, for example, atriac, a field-effect transistor (FET) in a rectifier bridge, or twoFETs in anti-series connection. The controllably conductive device 210includes a control input coupled to a drive circuit 212. Thecontrollably conductive device 210 is rendered conductive ornon-conductive in response to the control input, which in turn controlsthe power supplied to the lighting load 104.

The drive circuit 212 provides control inputs to the controllablyconductive device 210 in response to control signals from a controller214. The controller 214 is, for example, a microcontroller, but mayalternatively be any suitable processing device, such as a programmablelogic device (PLD), a microprocessor, or an application specificintegrated circuit (ASIC). The controller 214 receives inputs from thecontrol actuator 114 and the intensity adjustment actuator 116 andcontrols the visual indicators 118. The controller 214 is also coupledto a memory 216 for storage of the preset intensity of lighting load104, the serial number of the daylight sensor 120 to which the dimmerswitch 110 is assigned, the daylight gain G_(D), the electrical lightgain G_(E), and other operational characteristics of the dimmer switch110. The controller 230 may recall the daylight gain G_(D) and theelectrical light gain G_(E) from the memory 216 at startup. The memory216 may be implemented as an external integrated circuit (IC) or as aninternal circuit of the controller 214. A power supply 218 generates adirect-current (DC) voltage V_(CC) for powering the controller 214, thememory 216, and other low-voltage circuitry of the dimmer switch 110.

A zero-crossing detector 220 determines the zero-crossings of the inputAC waveform from the AC power supply 102. A zero-crossing is defined asthe time at which the AC supply voltage transitions from positive tonegative polarity, or from negative to positive polarity, at thebeginning of each half-cycle. The zero-crossing information is providedas an input to controller 214. The controller 214 provides the controlsignals to the drive circuit 212 to operate the controllably conductivedevice 210 (i.e., to provide voltage from the AC power supply 102 to thelighting load 104) at predetermined times relative to the zero-crossingpoints of the AC waveform using a phase-control dimming technique.

The dimmer switch 110 further comprises an RF transceiver 222 and anantenna 224 for receiving the RF signals 106 from the daylight sensor120. The controller 214 is operable to control the controllablyconductive device 210 in response to the messages received via the RFsignals 106. Examples of the antenna 224 for wall-mounted dimmerswitches, such as the dimmer switch 110, are described in greater detailin U.S. Pat. No. 5,982,103, issued Nov. 9, 1999, and U.S. Pat. No.7,362,285, issued Apr. 22, 2008, both entitled COMPACT RADIO FREQUENCYTRANSMITTING AND RECEIVING ANTENNA AND CONTROL DEVICE EMPLOYING SAME.The entire disclosures of both patents are hereby incorporated byreference.

FIG. 6A is a simplified block diagram of the daylight sensor 120. Thedaylight sensor 120 comprises a controller 230 that is responsive to aphotosensitive circuit 231, which includes the photosensitive diode 232.The cathode of the photosensitive diode 232 is coupled to the controller230 via a transimpedance amplifier 234, which operates as acurrent-to-voltage converter. The anode of the photosensitive diode 232is coupled to circuit common through a controllable switch 235, whichallows the controller 230 to enable and disable the photosensitivecircuit 231 (using a photosensitive circuit enable control signal V_(PS)_(—) _(ENABLE)) as will be described in greater detail below.

The photosensitive diode 232 conducts a photosensitive diode currentI_(PD) having a magnitude dependent upon the magnitude of the light thatshines on the photosensitive diode (i.e., the total light intensityL_(T-SNSR) as measured by the daylight sensor 120). The transimpedanceamplifier 234 provides the controller 230 with a total light intensitycontrol signal V_(TOT) representative of the total light intensityL_(T-SNSR). Specifically, the magnitude of the total light intensitycontrol signal V_(TOT) generated by the transimpedance amplifier 234 isdependent upon the magnitude of the current I_(PD) conducted by thephotosensitive diode 232, and thus the total light intensity L_(T-SNSR).The controller 230 comprises an analog-to-digital converter (ADC), suchthat the controller is operable to sample the total light intensitycontrol signal V_(TOT) to generate a total light intensity sampleS_(TOT). The controller 230 uses a sampling period T_(SMPL) of, forexample, approximately one second, such that the controller samples thetotal light intensity control signal V_(TOT) approximately once everysecond during normal operation of the daylight sensor 120.

The daylight sensor 120 further comprises an RF transceiver 236, whichis coupled to the controller 230 and an antenna 238. The controller 230is operable to cause the RF transceiver 236 to transmit digital messagesto the dimmer switch 110 via the RF signals 106 in response to themagnitude of the total light intensity control signal V_(TOT). Thecontroller 230 may also be operable to receive a digital message fromthe dimmer switch 110 or another remote control device, such as apersonal digital assistant (PDA), for configuring the operation of thedaylight sensor 120. The controller 230 provides the digital message tobe transmitted by the RF transceiver 236 and obtains received digitalmessages from the RF transmitter via an RF data control signal V_(RF)_(—) _(DATA). The controller 230 also is operable to enable and disablethe RF transceiver via an RF enable control signal V_(RF) _(—)_(ENABLE). Alternatively, the RF transceiver 236 of the daylight sensor120 could comprise an RF transmitter and the RF transceiver 222 of thedimmer switch 110 could comprise an RF receiver to allow for one-waycommunication between the daylight sensor and the dimmer switch. The RFtransmitter may comprise, for example, part number CC1150 manufacturedby Texas Instruments Inc.

The controller 230 of the daylight sensor 120 is also responsive to aplurality of actuators 240 (i.e., the calibration button 150, the testbutton 152, and the link button 154), which provide user inputs to thedaylight sensor for use during calibration of the daylight sensor. Thecontroller 230 is operable to control one or more LEDs 242 to illuminatethe lens 124 to thus provide feedback during calibration of the daylightsensor 120. A laser pointer circuit 244 is coupled to the controller 230and is responsive to light that shines through the laser-pointerreceiving opening 156 from a laser pointer. Specifically, the controller230 responds to an input from the laser pointer circuit 244 in the samemanner as an actuation of the calibration button 150. The controller 230is further coupled to a memory 246 for storing the operationalcharacteristics of the daylight sensor 120. The daylight sensor 120 alsocomprises a battery V1 that provides a battery voltage V_(BATT) (e.g.,approximately three volts) for powering the controller 230, thephotosensitive circuit 231, the RF transceiver 236, and the othercircuitry of the daylight sensor 120.

The controller 230 is operable to control the photosensitive circuit 231and the RF transceiver 236 in order to conserve battery power.Specifically, the controller 230 is operable to enable thephotosensitive circuit 231 (by closing the switch 235 via thephotosensitive circuit enable control signal V_(PS) _(—) _(ENABLE)) fora small time period T_(PD) (e.g., 50 msec) during each sampling periodT_(SMPL), such that that the photosensitive diode 232 only conductscurrent for a portion of the time during normal operation (e.g., 5% ofthe time). In addition, the controller 230 only enables the RFtransceiver 236 (via the RF enable control signal V_(RF) _(—) _(ENABLE))when required. As previously mentioned, the controller 230 only enablesthe RF transceiver 236 to transmit digital messages when needed, i.e.,using the variable transmission rate (as will be described in greaterdetail below with reference to FIG. 8). The controller 230 only enablesthe RF transceiver 236 to receive digital messages in response to thelaser pointer circuit 244 receiving light from a laser pointer throughthe laser-pointer receiving opening 156. When the photosensitive circuit231 and the RF transceiver 236 are disabled, the controller 230 isoperable to enter a sleep mode in which the controller consumes lesspower.

FIG. 6B is a simplified schematic diagram of the daylight sensor 120showing the transimpedance amplifier 234 and the laser pointer circuit244 in greater detail. The transimpedance amplifier 234 comprises anoperational amplifier (“op-amp”) U250 having a non-inverting inputterminal coupled to circuit common. A feedback resistor 8252 is coupledbetween an inverting input terminal and an output terminal of the op-ampU250. The output terminal of the op-amp U250 provides to the controller230 the total light intensity control signal V_(TOT), which has amagnitude that varies in response to the magnitude of the photosensitivediode current I_(PD). The cathode of the photosensitive diode 232 iscoupled to the inverting input terminal of the op-amp U250, such thatthe photosensitive diode current I_(PD) is conducted through thefeedback resistor R252. Thus, the magnitude of the total light intensitycontrol signal V_(TOT) is dependent upon the magnitude of thephotosensitive diode current I_(PD) and the resistance of the feedbackresistor R252. For example, the resistor R252 may have a resistance ofapproximately 300 kΩ, such that the magnitude of the total lightintensity control signal V_(TOT) ranges from approximately zero volts tothree volts as the light intensity shining directly on thephotosensitive diode 232 ranges from approximately zero lux to 1000 lux.

The transimpedance amplifier 234 further comprises a feedback capacitorC254 (e.g., having a capacitance of approximately 0.022 μF) forproviding some low-pass filtering, such that the magnitude of the totallight intensity control signal V_(TOT) is not responsive tohigh-frequency noise in the photosensitive diode current I_(PD). Inaddition, the op-amp U250 is powered from the battery V1 through alow-pass filter comprising a resistor 8256 (e.g., having a resistance ofapproximately 22Ω and a capacitor C258 (e.g., having a capacitance ofapproximately 0.1 μF). The low-pass filter prevents high-frequency noisethat may be coupled to the battery V1 from the RF transceiver 236 fromaffecting the operation of the photosensitive circuit 231.

The laser pointer circuit 244 comprises a laser-responsive element,e.g., a light-emitting diode (LED) D260. The LED D260 is positionedinside the daylight sensor 120 such that light from a laser pointer mayshine through the laser-pointer receiving opening 156 and onto the LED.The LED D260 may be a green LED, such that a laser current I_(LASER)conducted through the LED increases in magnitude when a green laserpointer is shined onto the LED. A resistor R262 is coupled between theanode of the LED D260 and circuit common and has, for example, aresistance of approximately 1 MΩ. A capacitor C264 is coupled inparallel with the resistor R262 and has, for example, a capacitance ofapproximately 0.01 μF. The junction of the LED D260 and the resistorR262 is coupled to the controller 230 through a capacitor C265 (e.g.,having a capacitance of approximately 0.22 μF) and a resistor 8266(e.g., having a resistance of approximately 10 kΩ). The junction of thecapacitor C265 and the resistor 8266 is coupled to circuit commonthrough a resistor (e.g., having a resistance of approximately 1 MΩ).When a laser pointer is shined onto the LED D260 and the laser currentI_(LASER) increases in magnitude, the voltage across the parallelcombination of the resistor R262 and the capacitor C264 also increasesin magnitude. Accordingly, the capacitor C265 conducts a pulse ofcurrent and the laser pointer control signal V_(LASER) also increases inmagnitude. The input of the controller 230 that receives the laserpointer control signal V_(LASER) is an interrupt pin, such that thecontroller 230 is operable to come out of sleep mode in response to thelaser pointer. The controller 230 may then be operable to enable the RFtransceiver 236 to receive a digital message as will be described ingreater detail below with reference to FIG. 10A.

According to the present invention, the daylight sensor 120 is operableto transmit digital messages to the dimmer switch 110 using the variabletransmission rate that is dependent upon the present change in the totallight intensity L_(T-SNSR) as measured by the daylight sensor 120. Thedaylight sensor 120 is operable to determine the total light intensityL_(T-SNSR) from the magnitude of the total light intensity controlsignal V_(TOT), and to only transmit one or more values representativeof the total light intensity L_(T-SNSR) (e.g., in foot-candles) to thedimmer switch 110 when the total light intensity L_(T-SNSR) has changedby at least a first predetermined percentage ΔS_(MAX1). Since the totallight intensity L_(T-SNSR) as measured by the daylight sensor 120changes throughout a typical day, the variable transmission rate alsochanges throughout the day (as shown in FIG. 3). The variabletransmission rate ensures that the daylight sensor 120 only transmitsdigital messages when needed (i.e., when the total light intensityL_(T-SNSR) is changing quickly, but not too quickly). Because thecontroller 230 is able to disable the photosensitive circuit 231 (byopening the switch 235 via the photosensitive circuit enable controlsignal V_(PS) _(—) _(ENABLE)), the daylight sensor 120 is able toconserve battery power by not transmitting digital messages to thedimmer switch 110 as often when the total light intensity L_(T-SNSR) isrelatively constant with respect to time.

FIG. 7 is a simplified flowchart of a transmission algorithm 300executed by the controller 230 of the daylight sensor 120 according tothe first embodiment of the present invention, such that the daylightsensor 120 transmits digital messages using the variable transmissionrate. The transmission algorithm 300 as shown in FIG. 7 is generalizedand specific embodiments are detailed below. According to the firstembodiment, the controller 230 collects a predetermined number N_(SMPL)of measurements of the total light intensity L_(T-SNSR) (e.g., ten)during consecutive non-overlapping time intervals (i.e., windows) thateach have a length equal to a predetermined time period T_(WIN) (i.e.,T_(WIN)=N_(SMPL)·T_(SMPL)). The controller 230 determines one or moreestimators from a previous time interval and uses the estimator toestimate one or more predicted light intensity values in the presenttime interval. At end of the present time interval, the controller 230determines whether a digital message including one or more valuesrepresentative of the total light intensity L_(T-SNSR) should betransmitted to the dimmer switch 110 in response to the error betweenthe measured light intensity values and the predicted light intensityvalues. The transmission algorithm 300 is executed with at a periodequal to the predetermined time period T_(WIN). Accordingly, the minimumtime period between transmissions by the daylight sensor 120 accordingto the first embodiment is equal to the predetermined time periodT_(WIN). For example, the predetermined time period T_(WIN) may beapproximately ten seconds, but may alternatively range fromapproximately five seconds to thirty seconds.

Referring to FIG. 7, the controller 230 first measures the predeterminednumber N_(SMPL) of new total light intensity values at step 310, andstores the measured total light intensity values at step 312. Next, thecontroller 230 determines the predicted light intensity value(s) at step314 using the estimator(s) determined during one of the previous timeintervals, and calculates an error between the measured total lightintensity values and the predicted total light intensity values at step316. If the error is outside of predetermined limits (i.e., is toogreat) at step 318, the controller 230 calculates the new estimator(s)for use during the subsequent time interval at step 320 and transmits adigital message including one or more values representative of the totallight intensity L_(T-SNSR) as measured by the daylight sensor 120 to thedimmer switch 110 at step 322. For example, the controller 230 maytransmit one or more of the measured total light intensity values to thedimmer switch 110. Alternatively, the controller 230 may transmit thenew estimator(s) determined at step 320 to the dimmer switch 110. Aftertransmitting a digital message to the dimmer switch 110, thetransmission algorithm 300 loops around, such that the controller 230may collect the predetermined number N_(SMPL) of measurements of thetotal light intensity L_(T-SNSR) during the subsequent non-overlappingtime interval. If the error is within the predetermined limits at step318, the controller 230 does not calculate the new estimator(s) at step320 and does not transmit the value representative of the total lightintensity L_(T-SNSR) at step 324, but simply analyzes the nextnon-overlapping time interval.

According to the first embodiment of the present invention, thecontroller 230 of the daylight sensor 120 uses a single data point asthe estimator. For example, the controller 230 may use the minimum valueof the measured light intensity values from the previous time intervalas the estimator. Alternatively, the controller 230 may use the averageor median value of the measured light intensity values from the previoustime interval as the estimator. Since the estimator is a single datapoint, the controller 230 only uses one predicted light intensity valueat step 314 of the transmission algorithm 300. For example, thepredicted light intensity value may be equal to the estimator. Thecontroller 230 then calculates the error using the minimum value of themeasured light intensity values from the present time interval and thepredicted light intensity value (i.e., the estimator).

FIG. 8 is a simplified flowchart of a variable transmission controlprocedure 400 executed by the controller 230 of the daylight sensor 120according to the first embodiment of the present invention. Thecontroller 230 executes the variable transmission control procedure 400periodically (e.g., approximately once every second) during normaloperation in order to sample the total light intensity control signalV_(TOT), to thus collect the predetermined number N_(SMPL) of samples(e.g., approximately ten samples) during each of the consecutivenon-overlapping time intervals. Specifically, the controller 230 firstenables the photosensitive circuit 231 at step 410 by closing thecontrollable switch 235 using the photosensitive circuit enable controlsignal V_(PS) _(—) _(ENABLE). The controller 230 waits for the timeperiod T_(PD) (i.e., 50 msec) at step 412 to allow the photosensitivediode current I_(PD) to become representative of the total lightintensity L_(T-SNSR) at the daylight sensor 120. The controller 230samples the total light intensity control signal V_(TOT) to generate anew total light intensity sample S_(TOT) at step 414, and disables thephotosensitive circuit 231 by opening the switch 235 using thephotosensitive circuit enable control signal V_(PS) _(—) _(ENABLE) atstep 416. The controller 230 then increments a variable n by one at step418 and stores the new total light intensity sample S_(TOT) as sampleS[n] in the memory 246 at step 420. If the variable n is less than thepredetermined number N_(SMPL) of samples at step 422, the variabletransmission control procedure 400 simply exits without processing thesamples S[n] stored in the memory 246. The controller 230 will executethe variable transmission control procedure 400 once again to collect anew sample of the total light intensity control signal V_(TOT).

If the variable n is greater than or equal to the predetermined numberN_(SMPL) of samples at step 422, the controller 230 processes thesamples S[n] stored in the memory 246 in order to determine if a digitalmessage should be transmitted to the dimmer switch 110. First, thecontroller 230 resets the variable n to zero at step 424. The controller230 then determines if the total light intensity L_(T-SNSR) has changedby at least the first predetermined percentage ΔS_(MAX1). Specifically,the controller 230 determines a present minimum sample S_(MIN-PRS) ofthe samples S[n] stored in the memory 246 (i.e., samples S[0] throughS[N_(SMPL)]) at step 426. The controller 230 then calculates a minimumsample adjustment percentage ΔS_(MIN) that is representative of theamount of change of the total light intensity L_(T-SNSR) at step 428using the equation:

$\begin{matrix}{{{\Delta \; S_{MIN}} = \frac{{S_{{MIN}\text{-}{PRS}} - S_{{MIN}\text{-}{PRV}}}}{S_{{MIN}\text{-}{PRV}}}},} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

where the sample S_(MIN-PRV) is the previous minimum sample determinedduring the previous time period T_(WIN) that is stored in the memory246. If the minimum sample adjustment percentage ΔS_(MIN) is less thanthe first predetermined percentage ΔS_(MAX1) at step 430, the variabletransmission control procedure 400 exits without the controller 230transmitting a digital message to the dimmer switch 110. In other words,the controller 230 has determined that the total light intensityL_(T-SNSR) has not changed significantly enough to merit a transmissionof a digital message. For example, the first predetermined percentageΔS_(MAX1) may be approximately 15%, but may alternatively range fromapproximately 1% to 20%.

If the minimum sample adjustment percentage ΔS_(MIN) is greater than orequal to the first predetermined percentage ΔS_(MAX1) at step 430, thecontroller 230 sets the previous minimum sample S_(MIN-PRV) equal to thepresent minimum sample S_(MIN-PRS) at step 432. The controller 230 thenloads a digital message including a value representative of the totallight intensity L_(T-SNSR) as measured by the daylight sensor 120 (e.g.,in foot-candles) in a transmit (TX) buffer at step 434, before thevariable transmission control procedure 400 exits. For example, thecontroller 230 may include the minimum present minimum sampleS_(MIN-PRS) in the digital message loaded into the TX buffer. Thecontroller 230 will transmit the digital message to the dimmer switch110 via the RF signals 106 using a transmit procedure (not shown). Anexample of a transmit procedure is described in previously-referencedU.S. patent application Ser. No. 12/203,518.

FIG. 9 is a simplified flowchart of a receive procedure 500 executed bythe controller 214 of the dimmer switch 110 when a digital message isreceived from the daylight sensor 120 at step 510. As previouslymentioned, the dimmer switch 110 adjusts the present light intensityL_(PRES) of the lighting load 104 so as to control the total lightintensity L_(T-TASK) on the task surface towards the target total tasksurface light intensity L_(TRGT-TASK). Specifically, the dimmer switch110 uses a present dimming percentage d_(PRES) to control the presentlight intensity L_(PRES) of the lighting load 104. The present dimmingpercentage d_(PRES) is calculated in response to the received digitalmessages and a target task surface light intensity value L_(TRGT-TASK)during the receive procedure 500. For example, the present dimmingpercentage d_(PRES) may be a number between zero and one. The controller214 may apply the present dimming percentage d_(PRES) to differentdimming curves depending upon the load type of the lighting load 104(i.e., incandescent, fluorescent, etc.) to determine the actual newpresent light intensity L_(PRES) of the lighting load.

Referring to FIG. 9, if the received digital message does not include alight intensity value received from the daylight sensor 120 at step 512,the controller 214 processes the digital message appropriately at step514 and the receive procedure 500 exits. For example, the digitalmessage may comprise a calibration message transmitted during acalibration procedure of the daylight sensor 120. However, if thereceived digital message includes a light intensity value at step 512,the controller 214 stores the total light intensity L_(T-SNSR) asmeasured by the daylight sensor 120 (and as received in the digitalmessage) in the memory 248 at step 516. As noted above, the valuerepresentative of the total light intensity L_(T-SNSR) in the receiveddigital message may be equal to the minimum present minimum sampleS_(MIN-PRS) from the variable transmission control procedure 400executed by the controller 230 of the daylight sensor 120 (i.e.,L_(T-SNSR)=S_(MIN-PRS)).

At step 518, the controller 214 calculates the light intensityL_(E-SNSR) measured by the daylight sensor 120 from only the lightingload 104 using the electric light gain G_(E), i.e.,

$\begin{matrix}{{L_{E\text{-}{SNSR}} = \frac{d_{PRES} \cdot L_{{EM}\text{-}{TASK}}}{G_{E}}},} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

where L_(EM-TASK) represents the light intensity on the task surface 136from only the lighting load 104 when the lighting load is at the maximumlight intensity. For example, the controller 214 may set the lightintensity L_(EM-TASK) from Equation 2 equal to the light intensityL_(E-TASK) on the task surface from only the lighting load 104 (from thegain calibration procedure), or to a predetermined value, such as, fiftyfoot-candles. At step 520, the controller 214 calculates the lightintensity L_(D-SNSR) at the daylight sensor 120 from only natural lightby subtracting the light intensity L_(E-SNSR) at the daylight sensorfrom only the lighting load 104 (as calculated at step 518) from thetotal light intensity L_(T-SNSR) measured by the daylight sensor (asreceived in the digital message), i.e.,

L _(D-SNSR) =L _(T-SNSR) −L _(E-SNSR)  (Equation 3)

At step 522, the controller 214 calculates the light intensityL_(D-TASK) on the task surface from only daylight by multiplying thelight intensity L_(D-SNSR) at the daylight sensor 120 from only daylightby the daylight gain G_(D), i.e.,

L _(D-TASK) =G _(D) ·L _(D-SNSR).  (Equation 4)

At step 524, the controller 214 calculates the new present dimmingpercentage d_(PRES) as a function of the target total task surface lightintensity L_(TRGT-TASK), the light intensity L_(D-TASK) on the tasksurface from only daylight, and the light intensity L_(EM-TASK) on thetask surface 136 from only the lighting load 104 when the lighting loadis at the maximum light intensity, i.e.,

$\begin{matrix}{d_{PRES} = {\frac{L_{{TRGT}\text{-}{TASK}} - L_{D\text{-}{TASK}}}{L_{{EM}\text{-}{TASK}}}.}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$

Finally, the controller 214 controls the lighting load 104 according tothe new present dimming percentage d_(PRES), before the receiveprocedure 500 exits.

FIG. 10A is a simplified flowchart of a laser pointer interruptprocedure 600 executed by the controller 230 in response to the laserpointer circuit 244 detecting light from a laser pointer when thecontroller is in the sleep mode. Specifically, when the laser pointercontrol signal V_(LASER) is pulled high towards the battery voltageV_(BATT) at step 610, the controller 230 first enables the RFtransceiver 236 via the RF enable control signal V_(RF) _(—) _(ENABLE)at step 612. The controller 230 then waits until a digital message isreceived at step 614 or a timeout expires at step 616. If a digitalmessage is received at step 614 (e.g., from the dimmer switch 110), thecontroller 230 stores the received digital message in a receive (RX)buffer at step 618, such that the controller 230 may process thereceived digital message at a later time. The controller 230 thendisables the RF transceiver 236 via the RF enable control signal V_(RF)_(—) _(ENABLE) at step 620, and the laser pointer interrupt procedure600 exits. If the timeout expires at step 616 before a digital messageis received at step 614, the controller 230 simply disables the RFtransceiver 236 at step 620, before the laser pointer interruptprocedure 600 exits.

FIG. 10B is a diagram of an example test setup 650 and FIG. 10C is aplot of an example test waveform 670 for the daylight sensor 120 of thefirst embodiment. The test setup 650 comprises test box 652 having afirst compartment in which a test lighting load 604 is located and asecond compartment in which the daylight sensor 120 and a light meter654 are located. The first and second compartments of the test box 652are separated by a light diffuser 656. The test setup 650 furthercomprises a personal computer (PC) 658, which coupled to the daylightsensor 120 and the light meter 654 via serial connections 662. The PC isoperable to adjust the intensity of the lighting load 604 using a dimmercircuit 660 according to the test waveform 670 and to observe thetransmission rate of the daylight sensor 120 as well as the actual lightintensity as measured by the light meter 654. The test waveform 670controls the intensity of the test lighting load 604 linearly from aminimum test intensity L_(T-MIN) to a maximum test intensity L_(T-MAX)and has a length T_(TEST), such that the test waveform 670 has a slopem_(TEST), i.e., m_(TEST)=(L_(T-MAX)−L_(T-MIN))/T_(TEST). If the testwaveform 670 has a first slope, the rate of transmission of the daylightsensor 120 of the first embodiment will remain constant at a first rate.For example, the minimum test intensity L_(T-MIN) may be zerofoot-candles, the maximum test intensity L_(T-MAX) may be 50foot-candles, and the length T_(TEST) may be two hours. If the testwaveform 670 is altered to have a second slope less than the first slope(e.g., if the length T_(TEST) is increased to approximately threehours), the rate of transmission of the daylight sensor 120 will changeto a second rate less than the first rate.

According to a second embodiment of the present invention, thecontroller 230 uses a linear least-squares prediction model to determinethe predicted light intensity values. Specifically, the controller 230is operable to perform a linear least-squares fit on the measured lightintensity values from a present time interval to determine a slope m andan offset b of a line (i.e., y=mx+b) that best represents the change inthe measured light intensity values with respect to time. The controller230 uses these estimators (i.e., the slope m and the offset b) todetermine the predicted light intensity values for one or more of thesubsequent time intervals. The controller 230 then determines amean-square error e between the measured light intensity values and thepredicted light intensity values.

FIG. 11 is a simplified flowchart of a variable transmission controlprocedure 700 executed by the controller 230 of the daylight sensor 120according to the second embodiment of the present invention. As in thefirst embodiment, the controller 230 executes the variable transmissioncontrol procedure 700 of the second embodiment periodically (e.g.,approximately once every second) during normal operation to sample thetotal light intensity control signal V_(TOT) and to collect thepredetermined number N_(SMPL) of samples during each of the consecutivenon-overlapping time intervals. The controller 230 closes thecontrollable switch 235 to enable the photosensitive circuit 231 at step710, and waits for the time period T_(PD) (i.e., 50 msec) at step 712 toallow the photosensitive diode current I_(PD) to become representativeof the total light intensity L_(T-SNSR). The controller 230 then samplesthe total light intensity control signal V_(TOT) (to generate a newtotal light intensity sample S_(TOT)) at step 714, and opens thecontrollable switch 235 to disable the photosensitive circuit 231 atstep 716. The controller 230 increments a variable n by one at step 718and stores the new total light intensity sample S_(T-SNSR) as sampleS[n] in the memory 246 at step 720. If the controller 230 has not yetcollected the predetermined number N_(SMPL) of samples during thepresent time interval at step 722, the variable transmission controlprocedure 700 simply exits without processing the samples S[n] stored inthe memory 246.

When the controller 230 has collected the predetermined number N_(SMPL)of samples during the present time interval at step 722, the controller230 processes the samples S[n] stored in the memory 246 to determine ifa digital message should be transmitted to the dimmer switch 110. Thecontroller 230 first increments a variable q at step 724. The controller230 uses the variable q to keep track of how many time intervals haveoccurred after the time interval in which the estimators were lastcalculated. The controller 230 then calculates the predicted lightintensity values at step 726 using the estimators (i.e., the slope m andthe offset b) from a previous time interval, i.e.,

P[i]=m·i+b,

for i=q·T _(WIN)+1 to 2q·T _(WIN).  (Equation 6)

At step 728, the controller 230 determines the mean-square error ebetween the measured light intensity values and the predicted lightintensity values, i.e.,

e=(1/N_(MAX))·Σ(S[i]−P[i])²,

for i=q*T _(WIN)+1 to 2q*T _(WIN).  (Equation 7)

If the mean-square error e is less than a predetermined maximum errore_(MAX) (e.g., approximately 15%) at step 730, the variable transmissioncontrol procedure 700 exits without transmitting a digital message tothe dimmer switch 110.

However, if the mean-square error e is greater than or equal to thepredetermined maximum error e_(MAX) at step 730, the controller 230 thendetermines the new estimators at step 732 by performing a linearleast-squares fit on the measured light intensities from the presenttime interval to thus determine the slope m and the offset b of the linethat best represents the measured light intensities from the presenttime interval. The controller 230 loads a digital message including oneor more values representative of the total light intensity L_(T-SNSR) inthe TX buffer at step 734. For example, the controller 230 may includethe estimators (i.e., the slope m and the offset b) determined at step732 in the digital message. Since the slope m and the offset bdetermined at step 732 represent the measured intensity values from thepresent time interval, the predicted intensity values determined in thenext subsequent time interval will begin at time T_(WIN), which is equalto the predetermined number N_(SMPL) of samples per interval. Therefore,the controller 230 resets the variable n to N_(SMPL) and the variable qto one at step 736, before the variable transmission control procedure700 exits.

Since both the slope m and the offset b as determined by the daylightsensor 120 are transmitted to the dimmer switch 110, the dimmer switchis operable to continuously re-calculate (i.e., estimate) the totallight intensity L_(T-SNSR) as a function of time, and to adjust thepresent light intensity L_(PRES) of the lighting load 104 in response tothe estimated total light intensity L_(T-SNSR). FIG. 12 is a simplifiedflowchart of a receive procedure 800 executed by the controller 214 ofthe dimmer switch 110 when a digital message is received from thedaylight sensor 120 at step 810 according to the second embodiment ofthe present invention. If the received digital message includes lightintensity values received from the daylight sensor 120 at step 812, thecontroller 214 stores the slope m and the offset b from the receiveddigital message in the memory 216 at step 814. The controller 214calculates the total light intensity L_(T-SNSR) as measured the daylightsensor 120 at step 816 using the slope m and the offset b from thereceived digital message, as well as the predetermined period T_(WIN) ofeach interval, i.e.,

L _(T-SNSR) =m·T _(WIN) +b.  (Equation 8)

The controller 214 then stores the calculated total light intensityL_(T-SNSR) in the memory 216 at step 818, before the receive procedure800 exits. If the received digital message does not include lightintensity values received from the daylight sensor 120 at step 812, thecontroller 214 processes the digital message appropriately at step 820and the receive procedure 800 exits.

FIG. 13 is a simplified flowchart of a load control procedure 900executed by the controller 214 of the dimmer switch 110 periodicallyaccording to an adjustment period T_(ADJ) (e.g., one second), such thatthe load control procedure 900 is executed once per second. Thecontroller 214 first updates the total light intensity L_(T-SNSR) (withrespect to time) at step 910 using the slope m stored in the memory 216,i.e.,

L _(T-SNSR) =L _(T-SNSR) +m·T _(ADJ).  (Equation 9)

The controller 214 then determines the new present dimming percentaged_(PRES) for the lighting load 104 in a similar manner as in the receiveprocedure 500 of the first embodiment. Specifically, the controller 214calculates the light intensity L_(E-SNSR) measured by the daylightsensor 120 from only the lighting load 104 at step 912, calculates thelight intensity L_(D-SNSR) at the daylight sensor 120 from only naturallight at step 914, calculates the light intensity L_(D-TASK) on the tasksurface from only daylight at step 916, and calculates the new presentdimming percentage d_(PRES) at step 918. The controller 214 then finallycontrols the lighting load 104 according to the new present dimmingpercentage d_(PRES) at step 920, before the load control procedure 900exits.

According to a third embodiment of the present invention, the controller230 uses a parabolic model to determine the predicted light intensityvalues. In other words, the controller 230 is operable to perform aparabolic least-squares fit on the measured light intensity values froma present time interval to fit measured light intensity values to aparabola (i.e., y=ax²+bx+c) that best represents the change in themeasured light intensity values with respect to time. The controller 230uses these estimators (i.e., the coefficients a, b, c of the parabola)to determine the predicted light intensity values for one or more of thesubsequent time intervals. The controller 230 then determines amean-square error e between the measured light intensity values and thepredicted light intensity values.

FIG. 14 is a simplified flowchart of a variable transmission controlprocedure 1000 executed by the controller 230 of the daylight sensor 120periodically (e.g., approximately once every second) according to thethird embodiment of the present invention. The variable transmissioncontrol procedure 1000 is very similar to the variable transmissioncontrol procedure 700 of the second embodiment. However, the controller230 calculates the predicted light intensity values at step 1026 usingthe coefficients a, b, c (i.e., the estimators) and the parabolaequation, i.e.,

P[i]=ai ² +bi+c,

for i=q·T _(WIN)+1 to 2q·T _(WIN).  (Equation 10)

At step 1028, the controller 230 determines the mean-square error ebetween the measured light intensity values and the predicted lightintensity values. If the mean-square error e is greater than or equal tothe predetermined maximum error e_(MAX) at step 1030, the controller 230determines the new estimators at step 1032 by performing a parabolicleast-squares fit on the measured light intensities from the presenttime interval to thus determine the coefficients a, b, c of the parabolathat best represent the measured light intensities from the present timeinterval. The controller 230 then loads a digital message including oneor more values representative of the total light intensity L_(T-SNSR) inthe TX buffer at step 1034, e.g., the estimators (i.e., the coefficientsa, b, c of the parabola) determined at step 1032. Accordingly, thedimmer switch 110 will execute a receive procedure (not shown) similarto the receive procedure 800 of the second embodiment in order tocalculate the total light intensity L_(T-SNSR) as measured by thedaylight sensor 120 using the coefficients a, b, c. In addition, thedimmer switch 110 will periodically adjust the present light intensityL_(PRES) of the lighting load 104 using a load control procedure (notshown) similar to the load control procedure 900 of the secondembodiment.

According to another alternative embodiment of the present invention,the controller 230 of the daylight sensor 120 could use a linearpredictor to determine the predicted light intensity values. Forexample, the predicted light intensity values may be calculated usingthe equation:

P[i]=−Σ(α ₁ ·x[n−i])

for i=1 to K,  (Equation 11)

where x[n−i] are the previous measured light intensity values, α₁ arethe predictor coefficients, and K is the maximum number of values usedto calculate the predicted light intensity.

According to a fourth embodiment of the present invention, the daylightsensor 120 does not transmit digital messages in response to themeasured total light intensity L_(T-SNSR) if the measured data is“misbehaving” so as to reduce the transmission rate and further conservebattery life. For example, the daylight sensor 120 may ignorefluctuations in the measured total light intensity L_(T-SNSR) that arelarge in magnitude and short in time duration (i.e., duringintermittent-cloudy days as shown in FIG. 3), such that the variabletransmission rate of the daylight sensor is also dependent upon the rateof change of the total light intensity L_(T-SNSR) measured by thedaylight sensor (i.e., the “dynamic” change in the total lightintensity). Specifically, the daylight sensor 120 does not transmitdigital messages to the dimmer switch 110 if the total light intensityL_(T-SNSR) has changed by more than a second predetermined percentageΔS_(MAX2) during the predetermined time period T_(WIN). Accordingly, thevariable transmission rate of the daylight sensor 120 of the fourthembodiment of the present invention results in the average time betweentransmissions by the daylight sensor during the course of a day beinggreater than approximately 420 seconds (as determined by experimentalstudy).

FIG. 15 is a simplified flowchart of a transmission algorithm 1100executed by the controller 230 of the daylight sensor 120 according tothe fourth embodiment of the present invention, such that the daylightsensor 120 transmits digital messages using the variable transmissionrate. The transmission algorithm 1100 of the fourth embodiment issimilar to the transmission algorithm 300 of the first, second, andthird embodiments (as shown in FIG. 7). The controller 230 firstmeasures and stores the predetermined number N_(SMPL) of new total lightintensity values at steps 310 and 312. Next, the controller 230determines the predicted light intensity value(s) at step 314 using, forexample, any of the estimators described with reference to the firstthrough third embodiments, and calculates the error between the measuredtotal light intensity values and the predicted total light intensityvalues at step 316.

However, according to the fourth embodiment, the controller 230 furtheranalyzes the measured total light intensity values if the errorcalculated at step 316 is outside of the predetermined limits (i.e., istoo great) at step 318. Specifically, the controller 230 using themeasured total light intensity values to calculate a data behaviormetric at step 1124, compares the calculated data behavior metric topredetermined data behavior metric limit(s) at step 1126, and determinesif the data is misbehaving at step 1128, i.e., is outside of the databehavior metric limit(s). For example, the controller 230 may analyzethe total light intensity values to determine if the rate of change ofthe total light intensity L_(T-SNSR) measured by the daylight sensor 120is too great. If the data is not misbehaving at step 1128, thecontroller 230 calculates the new estimator(s) for use during thesubsequent time interval at step 320 and transmits a digital messageincluding one or more values representative of the total light intensityL_(T-SNSR) as measured by the daylight sensor 120 to the dimmer switch110 at step 322, before the transmission algorithm 1100 loops around. Ifthe data is misbehaving at step 1128, the controller 230 does notcalculate the new estimator(s) at step 320 and does not transmit thevalues representative of the total light intensity L_(T-SNSR) at step324, but simply analyzes the next non-overlapping time interval.

FIG. 16A is a simplified flowchart of a variable transmission controlprocedure 1200 executed by the controller 230 of the daylight sensor 120periodically (e.g., approximately once every second) according to thefourth embodiment of the present invention. The variable transmissioncontrol procedure 1200 of the fourth embodiment is very similar to thevariable transmission control procedure 400 of the first embodiment (asshown in FIG. 8). According to the fourth embodiment of the presentinvention, the controller 230 uses a single data point as the estimator(as in the first embodiment). However, the controller 230 couldalternatively use a linear prediction model or a parabolic predictionmodel to determine the estimators as described above with reference tothe second and third embodiments respectively.

Referring to FIG. 16A, if the minimum sample adjustment percentageΔS_(MIN) is greater than or equal to the first predetermined percentageΔS_(MAX1) at step 430, the controller 230 determines if the data (i.e.,the samples S[n] stored in the memory 246) is misbehaving by determiningif the total light intensity L_(T-SNSR) has changed by more than thesecond predetermined percentage ΔS_(MAX2) during the present time periodT_(WIN). Specifically, the controller 230 determines a present maximumsample S_(MAX-PRS) of the samples S[n] stored in the memory 246 (i.e.,samples S[0] through S[N_(SMPL)]) at step 1236. The controller 230 thencalculates a present sample adjustment amount ΔS_(PRS), which isrepresentative of the rate of change of the total light intensityL_(T-SNSR), at step 1238 using the equation:

$\begin{matrix}{{\Delta \; S_{PRS}} = {\frac{S_{{MAX}\text{-}{PRS}} - S_{{MIN}\text{-}{PRS}}}{S_{{MAX}\text{-}{PRS}}}.}} & \left( {{Equation}\mspace{14mu} 12} \right)\end{matrix}$

If the present sample adjustment amount ΔS_(PRS) is greater than orequal to the second predetermined percentage ΔS_(MAX2) at step 1240, thevariable transmission control procedure 1200 exits without transmittinga digital message to the dimmer switch 110. For example, the secondpredetermined percentage ΔS_(MAX2) may be approximately 10%, but mayalternatively range from approximately 5% to 25%.

However, if the present sample adjustment amount ΔS_(PRS) is less thanthe second predetermined percentage ΔS_(MAX2) at step 1240, thecontroller 230 sets the previous minimum sample S_(MIN-PRV) equal to thepresent minimum sample S_(MIN-PRS) at step 432. The controller 230 thenloads a digital message including a value representative of the totallight intensity L_(T-SNSR) as measured by the daylight sensor 120 (i.e.,the minimum present minimum sample S_(MIN-PRS)) in a transmit (TX)buffer at step 434, before the variable transmission control procedure1200 exits.

FIG. 16B is a plot of an example test waveform 1250 for the daylightsensor 120 of the fourth embodiment to be used in the test setup 650shown in FIG. 10B. The test waveform 1250 comprises a pulsed waveformadded on top of a linear ramp waveform and has peaks and valleys, suchthat the text waveform models the total light intensity L_(T-SNSR) asmeasured by the daylight sensor 120 on an intermittent-cloudy day. Thetest waveform 1250 has a minimum light intensity (during the valleys)that increases with respect to time at a first slope m₁, and a maximumlight intensity (during the peaks) that increases with respect to timeat a second slope m₂. Each of the peaks (during which the text waveform1250 is at the maximum light intensity) have a length T_(PULSE), whichmay be approximately five seconds. The magnitude of the test waveform1250 during the valleys is approximately 15% of the magnitude of thetest waveform during the peaks. When the test waveform 1250 is used inthe test setup 650 to control the test lighting load 604, the daylightsensor 120 of the fourth embodiment will not transmit digital messagesin response to the temporary excursions of the light intensity duringthe peaks. Accordingly, the rate of transmission of the daylight sensor120 of the fourth embodiment will remain constant at a rate determinedby the slope m₁ of the valleys.

As described above, the controller 230 of the daylight sensor 120 of thefirst, second, third, and fourth embodiments collects the predeterminednumber N_(SMPL) of measurements of the total light intensity L_(T-SNSR)during consecutive non-overlapping time intervals, and only analyzes themeasurements at the end of each time interval (i.e., as determined bythe predetermined time period T_(WIN)). Alternatively, the controller230 could analyze the measurements of the total light intensityL_(T-SNSR) in a sliding window time interval. Specifically, thecontroller 230 could store each new measurement of the total lightintensity L_(T-SNSR) in a first-in, first-out (FIFO) register (e.g.,having a size equal to the predetermined number N_(SMPL) ofmeasurements). The controller 230 could then analyze the data stored inthe FIFO registered each time that the controller samples the totallight intensity control signal V_(TOT).

In addition, the controller 230 of the daylight sensor 120 transmitsdigital messages including one or more values representative of themeasured total light intensity L_(T-SNSR) according to the first,second, third, and fourth embodiments. According to a fifth embodimentof the present invention, each digital message transmitted by thedaylight sensor 120 to the dimmer switch 110 may alternatively comprisea command, such as a specific new light intensity L_(NEW) for thelighting load 104. The controller 230 of the daylight sensor 120determines the new intensity levels L_(NEW) in response to the measuredtotal light intensity L_(T-SNSR). The dimmer switch 110 controls thepresent light intensity L_(PRES) of the lighting load 104 to the newlight intensity L_(NEW) in response to receiving a digital message witha command from the daylight sensor 120.

According to the fifth embodiment, each time the controller 230 of thedaylight sensor 120 samples the total light intensity control signalV_(TOT), the controller 230 calculates a new dimming percentage d_(NEW),which may be transmitted to the dimmer switch 110. As in the previousembodiments, the new dimming percentage d_(NEW) may be a number betweenzero and one, which is representative of the new light intensity L_(NEW)for the lighting load 104. The controller 214 of the dimmer switch 110is operable to determine the light intensity L_(NEW) from the newdimming percentage d_(NEW) received from the daylight sensor 120, forexample, by applying the new dimming percentage d_(NEW) to differentdimming curves depending upon the load type of the lighting load. Thecontroller 230 of the daylight sensor 120 only transmits digitalmessages to the dimmer switch 110 when the new dimming percentaged_(NEW) is outside a deadband, i.e., only when a change to the presentlight intensity L_(PRES) of the lighting load 104 is required.Accordingly, the daylight sensor 120 only transmits digital messages tothe dimmer switch 110 using a variable transmission rate that isdependent upon the measured total light intensity L_(T-SNSR).

In addition, the controller 230 may also store a historical record ofthe total light intensity L_(T-SNSR) as measured by the daylight sensor120 each time the controller samples the total light intensity controlsignal V_(TOT). The controller 230 is operable to determine when it isdaytime and nighttime in response to the total light intensity controlsignal V_(TOT) and the historical record stored in the memory 246. Thecontroller 230 may increase the length of the sampling period T_(SMPL)(e.g., to approximately three seconds) during the nighttime, such thatthe controller samples the total light intensity control signal V_(TOT)less frequently and consumes even less power.

FIG. 17 is a simplified flowchart of a control procedure 1300 executedperiodically (e.g., every one to three seconds) by the controller 230 ofthe daylight sensor 120 according to the fifth embodiment of the presentinvention. At step 1310, the controller 230 enables the photosensitivecircuit 231 by closing the switch 235 using the photosensitive circuitenable control signal V_(P) _(—) _(ENABLE). The controller 230 waits forthe time period T_(PD) (i.e., 50 msec) at step 1312 to allow thephotosensitive diode current I_(PD) to become representative of thetotal light intensity L_(T-SNSR) at the daylight sensor 120. Thecontroller 230 then samples the total light intensity control signalV_(TOT) (using the ADC) to generate a new total light intensity sampleS_(TOT) at step 1314, and disables the photosensitive circuit 231 byopening the switch 235 using the photosensitive circuit enable controlsignal V_(PS) _(—) _(ENABLE) at step 1316. At step 1318, the total lightintensity sample S_(TOT) is applied to a digital filter (such as alinear predictor) to generate a filtered total light intensity sampleFS_(TOT).

The controller 230 is operable to periodically store the filtered totallight intensity samples FS_(TOT) (e.g., every 30 minutes) to create thehistorical record in the memory 246 of the total light intensityL_(T-SNSR) at the daylight sensor 120. Specifically, if the controller230 should store the present filtered total light intensity sampleFS_(TOT) at step 1320, the controller stores the present filtered totallight intensity sample FS_(TOT) in the memory 246 at step 1322.

Next, the controller 230 uses the filtered total light intensity sampleFS_(TOT) and a present dimming percentage d_(PRES) to determine the newdimming percentage d_(NEW) for the lighting load 104 using similarcalculations as the receive procedure 500 of the first embodiment.Specifically, the controller 230 calculates the light intensityL_(E-SNSR) measured by the daylight sensor 120 from only the lightingload 104 at step 1324, calculates the light intensity L_(D-SNSR) at thedaylight sensor 120 from only natural light at step 1326, calculates thelight intensity L_(D-TASK) on the task surface from only daylight atstep 1328, and calculates the new dimming percentage d_(NEW) at step1330.

At step 1332, the controller 230 determines if the new dimmingpercentage d_(NEW) is outside of a deadband, e.g.,

d _(PRES) −Δ<d _(NEW) <d _(PRES)+Δ,  (Equation 13)

where Δ represents a predetermined increment by which the new dimmerpercentage d_(NEW) must differ from the present dimming percentaged_(PRES) before the daylight sensor 120 will transmit a digital messageto the dimmer switch 110 causing the dimmer switch to adjust theintensity of the lighting load 104 to the new intensity L_(NEW). Forexample, the predetermined increment Δ may be approximately 1%. If thenew dimming percentage d_(NEW) is within the deadband at step 1332, thecontrol procedure 1300 simply exits. However, if the new dimmingpercentage d_(NEW) is outside the deadband at step 1332, the controller230 stores the new dimming percentage d_(NEW) as the present dimmingpercentage d_(PRES) at step 1334. The controller 230 loads a digitalmessage (including a command to control the intensity of the lightingload 104 according to the new dimming percentage d_(NEW)) into atransmit (TX) buffer at step 1336, before the control procedure 1300exits.

A lighting control systems including wired daylight sensors (i.e., wiredphotosensors) is described in greater detail in U.S. Pat. No. 7,369,060,issued May 6, 2008, entitled DISTRIBUTED INTELLIGENCE BALLAST SYSTEM ANDEXTENDED LIGHTING CONTROL PROTOCOL, the entire disclosures of which ishereby incorporated by reference.

While the present invention has been described with reference to thedimmer switch 110 for controlling the intensity of the lighting load104, the concepts of the present invention could be applied to loadcontrol systems comprising other types of load control devices, such as,for example, fan-speed controls for fan motors, electronic dimmingballasts for fluorescent loads, and drivers for light-emitting diodes(LEDs). Further, the concepts of the present invention could be used tocontrol other types of electrical loads, such as, for example, fanmotors or motorized window treatments.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

What is claimed is:
 1. A wireless sensor comprising: a photosensitivecircuit operable to generate a light intensity control signal inresponse to a light intensity; a wireless transmitter for transmittingwireless signals; and a controller coupled to the photosensitive circuitfor periodically sampling the light intensity control signal to generatesampled light intensity values, the controller further coupled to thewireless transmitter for transmitting a wireless signal in response tothe light intensity control signal, the wireless signal including avalue representative of the light intensity; wherein the controller isconfigured to analyze the sampled light intensity values to determine anamount of change of the light intensity, the controller configured totransmit the wireless signals using a variable transmission rate that isdependent upon the amount of change of the light intensity.
 2. Thesensor of claim 1, wherein the controller determines at least onepredicted light intensity value and calculates an error between thesampled light intensity values and the at least one predicted lightintensity value, the controller operable to transmit the wireless signalif the error is too great.
 3. The sensor of claim 2, wherein thecontroller collects a predetermined number of sampled light intensityvalues during consecutive non-overlapping time intervals, the controlleroperable to analyze the sampled light intensity values of each timeinterval to determine the amount of change of the light intensity. 4.The sensor of claim 3, wherein the controller determines at least oneestimator during a previous time interval, and uses the at least oneestimator to determine the at least one predicted light intensity valueduring a present time interval.
 5. The sensor of claim 4, wherein thecontroller calculates multiple predicted light intensity values duringthe present time interval.
 6. The sensor of claim 5, wherein thecontroller uses a linear prediction model to calculate the multiplepredicted light intensity values, the estimators comprising a slope andan offset of a line that best represents the change of the lightintensity.
 7. The sensor of claim 6, wherein the controller transmitsthe slope and the offset of the line that best represents the change ofthe light intensity when the error between the sampled light intensityvalues and the predicted light intensity values is too great.
 8. Thesensor of claim 5, wherein the controller uses a parabolic predictionmodel to calculate the multiple predicted light intensity values, theestimators comprising coefficients of a parabola that best representsthe change of the light intensity.
 9. The sensor of claim 5, wherein thecontroller calculates a mean-square error between the predicted lightintensity values and the sampled light intensity values, the controlleroperable to transmit a wireless signal if the mean-square error exceedsa maximum error.
 10. The sensor of claim 4, wherein the estimatorcomprises one of: (1) a minimum sampled light intensity value from theprevious time interval; (2) an average value of the sampled lightintensity values from the previous time interval; and (3) a median valueof the sampled light intensity values from the previous time interval;and wherein the controller is operable to transmit a wireless signalincluding the estimator if the difference between the estimator and theestimator exceeds a maximum error.
 11. The sensor of claim 1, whereinthe variable transmission rate is also dependent upon a rate of changeof the light intensity whereby the controller does not transmit thewireless signal if the rate of change is outside of predeterminedlimits.
 12. The sensor of claim 1, further comprising: a battery forpowering the photosensitive circuit, the wireless transmitter, and thecontroller.
 13. The sensor of claim 1, further comprising: a memorycoupled to the controller for storing the sampled light intensityvalues.
 14. A method of transmitting a wireless signal in response to alight intensity, the method comprising: generating a light intensitycontrol signal in response to the light intensity; periodically samplingthe light intensity control signal to generate sampled light intensityvalues; analyzing the sampled light intensity values stored in thememory to determine an amount of change of the light intensity; andtransmitting wireless signals using a variable transmission rate that isdependent upon the amount of change of the light intensity in the space,each digital message including a value representative of the lightintensity.
 15. The method of claim 14, further comprising: determiningat least one predicted light intensity value; and calculating an errorbetween the sampled light intensity values and the at least onepredicted light intensity value; wherein the controller is operable totransmit a wireless signal if the error is too great.
 16. The method ofclaim 15, further comprising: collecting a predetermined number ofsampled light intensity values during consecutive non-overlapping timeintervals; wherein the step of analyzing further comprises analyzing thesampled light intensity values of each time interval to determine theamount of change of the light intensity in the space.
 17. The method ofclaim 16, further comprising: determining at least one estimator duringa previous time interval; wherein the step of determining at least onepredicted light intensity value comprises using the at least oneestimator to determine multiple predicted light intensity values duringa present time interval.
 18. The method of claim 17, further comprising:calculating a mean-square error between the predicted light intensityvalues and the sampled light intensity values; wherein the step oftransmitting wireless signals comprises transmitting a wireless signalif the mean-square error exceeds a maximum error.
 19. The method ofclaim 14, wherein the variable transmission rate is also dependent upona rate of change of the light intensity whereby the controller does nottransmit a wireless signal if the rate of change is outside ofpredetermined limits.
 20. The method of claim 14, further comprising:storing sampled light intensity values in a memory after the step ofperiodically sampling the light intensity control signal.